1. Field of the Invention
This invention relates to a method of making 5 magnetic materials and devices therefrom.
2. Description of the Prior Art
Magnetic materials that are of commercial significance typically fall into two broad categories: ferromagnetic and ferrimagnetic. The so-called "soft" ferromagnetic materials are typically characterized by high permeability and low resistivity. The ferrimagnetic materials, on the other hand, tend to have somewhat lower permeabilities but significantly higher resistivities due to their oxide form. For higher frequency applications, the ferrimagnetic materials are often chosen, as their high resistivities result in low eddy current losses in devices made therefrom.
If ferromagnetic materials are to be used in certain applications, including high frequency applications, steps are typically taken to reduce the eddy current losses in such materials. For example, it is known that magnetic devices made by powder metallurgy techniques have somewhat lower eddy current losses than parts cast from a melt, due to the greater porosity and hence higher resistivity of the powder metallurgy material. A method of further increasing the resistivities of powder metallurgy materials which have inherently low resistivities, such as nickel, iron, cobalt, etc., is to coat the metal powder particles with an insulting material prior to compacting the powder and sintering. Typical insulating materials that have been used include colloidal clay, kaolin, milk of magnesia, and sodium silicate. Another known technique to increase resistivity is to oxidize the surface of metal powder grains before compaction. In addition, steps are typically taken to minimize the size of the metal powder particles, as smaller grain sizes typically result in lower eddy current losses. These steps typically include adding a small amount of sulfur to a melt of the magnetic metals, in order to embrittle the resultant metal. This allows smaller powder particles to be obtained upon grinding the metal, and may increase the resistivity of the particles. Other techniques to minimize eddy current losses, particularly in powder transformers, include making the devices of thin layers or laminations that are insulated from each other, or by choosing an alloy with a high electrical resistivity, such as silicon steel.
It would be desirable to find additional methods for obtaining low loss soft ferromagnetic materials for a wide variety of applications.